The invention relates to a processing technique for forming elliptic or similarly shaped Fresnel steps having a refractive-type and a reflecting-type prism, and relates further to a vehicle lamp lens having a Fresnel lens having such elliptic or similarly shaped Fresnel steps (hereinafter referred to as "an elliptic Fresnel lens").
Conventionally, Fresnel steps having refractive-type and reflecting-type prisms and formed on a lens plate of a Fresnel lens have generally had a front shape defined by a series of concentric circles, as shown in FIG. 9. Lens plates of curved shape have also been known, generally in the form of spherical surfaces. In either the case where the lens plate is a flat plate or a spherical plate having a radius of curvature the axis CL of rotation perpendicular to the surface at the center C coincides with the optical axis L (FIGS. 11(a) and 11(b)). Thus, lens plates have been limited to shapes consisting of a body of rotation about an axis. With such construction, the light distribution of each Fresnel step can be easily achieved by a two-dimensional design in the radial direction, and processing (working) and molding are easily carried out. In the case when the optical axis L is to be inclined with respect to the plate (dotted lines in FIGS. 11), on the other hand, the rotational axis CL can not be used for the light distribution design, such that a simple two-dimensional design can not be used.
In recent years, moreover, in order to reduce the air resistance of the outer shape of vehicle bodies, there have been increasingly employed lenses for automobile tail lamps and other vehicle lighting which lenses have a narrow rectangular outer shape. Further, there has been a demand for lenses defined by a quadric surface having different radii of curvature in two directions (i.e., having no axis of rotation or revolution). Therefore, conventionally, in molding a rectangular lens member for a vehicle lamp lens, a core 53 for forming a (flat) Fresnel lens portion 50 has been fitted in a mold 52 for forming a lattice-like prism portion 51. For the latter, lattice-like Fresnel steps (angular linearly-transmitting steps) have been used as shown in FIG. 12. However, with such construction, the processing steps are increased, which directly causes increased costs. Thus, this is undesirable from an economical point of view. Furthermore, in vehicle lamp lenses of such type, generally, the molding pattern of the Fresnel lens portion 50 is different from that of the prism portion 51, thus providing a discontinuous pattern. Therefore, the design of the pattern is limited, and the mold has poor cooling characteristics because of the use of the core, which results in more defective moldings.